1. Field of the Invention
The present invention relates to a focus detection apparatus for detecting a focusing state of an objective lens wherein light projects on an object and light reflected by the object is detected through the objective lens.
2. Description of the Prior Art
A variety of conventional active type focus detection apparatuses have been proposed. For example, a conventional apparatus is proposed wherein light irradiates an object through an objective lens subjected to focus detection and reflected light from the object is detected through the objective lens. A conventional TTL (through-the-lens) active type focus detection apparatus will be described with reference to FIGS. 1A to 1C. A light source 1, a light projecting lens 2, a light receiving lens 4 and a sensor 5 are disposed in a predetermined relationship behind a predetermined imaging surface (a plane conjugate with a film surface of a camera when a given apparatus is built into the camera) of an objective lens (an objective lens when the given apparatus is built into the camera) 3 to be focused on an object S. The light projecting lens 2 and the light receiving lens 4 have optical axes O1 and O2 deviated from an optical axis O of the objective lens 3 and parallel thereto. The light projecting lens 2 is substantially symmetrical with the light receiving lens 4 with respect to the optical axis O. Light rays L1 emitted from the light source 1 are firstly focused by the projecting lens 2 on an imaging plane F and are projected on the object S through the objective lens 3. Light rays L2 reflected by the object S are firstly focused in the vicinity of the plane F through the objective lens 3. Thereafter, the light rays are secondarily focused by the receiving lens 4 to become incident on the sensor 5.
In this apparatus, when the objective lens 3 is in an in-focus state with respect to the object S, the light rays L1 radiate the center of the object S, as shown in FIG. 1B, and the reflected light rays L2 are secondarily focused by the objective lens 3 and the light receiving lens 4 at the center of the sensor 5. However, when the objective lens 3 is shifted to be out of focus, light rays L1 are shifted above or below the center of the object S in accordance with a near-focus state and a far-focus state illustrated in FIGS. 1A and 1C, respectively. Therefore, when the reflected rays L2 are received by the sensor 5 through the objective lens 3 and the light receiving lens 4, the imaging position is also moved upward or downward. The sensor 5 comprises a differential sensor consisting of light-receiving elements 5a and 5b. The amount of light received by the light-receiving element 5a is compared with that received by the light-receiving element 5b, thereby detecting the light-receiving position. The focusing state of the objective lens 3 is detected to be one of the in-, far- and near-focus states.
In the TTL active type focus detection apparatus of this type, light projection and light reception are performed through the objective lens 3 subjected to focusing. In this manner, a parallax does not occur and objective lenses can be interchanged. In addition, when the brightness of the object S is low, the detection apparatus can be effectively used, resulting in convenience.
However, this conventional detection apparatus has the following disadvantage. The apparatus can be properly operated only when the objective lens 3 is set in the in-focus state or is located in the vicinity of the in-focus position. As shown in FIG. 1D, when the defocusing amount of the objective lens 3 is increased, a part L2' of the reflected light rays L2 are subjected to eclipse by the peripheral portion of the light receiving lens 4. The amount of light reaching the sensor 5 is thus decreased. As a result, it becomes difficult to discriminate between the near- or far-focus state. This tendency typically occurs when the F-number of the objective lens 3 or the light receiving lens 4 is large. In addition, when the objective lens comprises an objective lens 3 having a long focal length, the defocusing amount is increased, and a displacement of the firstly focusing point in a direction perpendicular to the optical axis is increased. In the worst case, the reflected light rays L2 will not reach the sensor 5 at all.
When the focusing state of the objective lens of the camera is to be detected, the light-source 1, the light projecting lens 2, the light receiving lens 4 and sensor 5 must be arranged in the camera body. These components cannot be extremely minimized in size due to the optical characteristic and manufacturing limitations. As a result, a large space is required to arrange these components. This causes a problem typically in a single lens reflex camera because it is difficult to assure a large space for incorporating the above-mentioned components in this type of camera since mechanical, electrical and optical components are incorporated in the camera body.